1. Field of the Invention
This invention relates to photodetection systems and related circuitry which, upon the occurrence of a specified event, generate an output control signal. In particular, this invention relates to a system of interrelated electrical, optical, and mechanical modules designed and deployed so as to sense the intrusion of an opaque object into a controlled space and upon such sensing to immediately initiate a machine-stopping or machine-starting sequence. More particularly, this invention relates to improved light curtain systems manifesting increased reliability and accuracy and imperviousness to spurious signals. More particularly yet, this invention relates to a microcontroller-based light curtain system containing an integrated collection of improvements over traditional light curtain systems. These improvements include provisions for continual self-testing during operation--including self-monitoring for deliberately simulated false events--as well as the means to react to detected defects in a manner safeguarding those depending on the safety switch nature of the light curtain.
2. Description of the Prior Art
A photodetection system--or "electric eye"--plays the role of a switch. In general, it does not by itself cause things to happen, but it generates a control signal that triggers other mechanisms. When the system creates a planar light wall over an extended plane--as opposed to a single narrow beam of light--it is referred to as a "light curtain." This type of photodetection system is then an area light switch, since it generates an output control signal whenever anything opaque to the light penetrates the area defined by the light.
Although its use in stopping fast machinery is of relatively recent origin, the basic electric eye has been used throughout most of this century in one form or another. For a minimal light curtain, all that is needed is an extended light source directed toward an extended light detector, with the latter wired so as to provide an output signal related to the light flux incident on it. This minimal setup was sufficient for thousands of different types of light-based control systems developed during the past 60-70 years, control systems ranging from the mundane (e.g., grocery store door-openers) through the exotic (e.g., marking the passage of short-lived elementary particles) to the critical (e.g., slit-second deactivation of fast machinery). For some of these applications--and especially the last-mentioned--it is very important that the control system give a positive response every single time that the event-to-be-detected occurs and that it never give a positive response in the absence of that event. Light curtain applications demanding extreme speed coupled with great dependability can be implemented only after many problems have been resolved. These problems arise in part because it is a balanced dependability which is sought; not only must the light curtain switch always send out the appropriate control signal when the triggering condition is present, but it must never send out that signal when that condition is not present. (It is fairly easy to deliver a circuit which will always respond to the triggering condition, providing that no difficulties arise when the circuit stops the machine occasionally in the absence of the triggering condition. Similarly, it is easy to deliver a circuit which will never generate a control signal in the absence of the triggering condition, but at the cost of occasional failures to generate a control signal when the triggering condition is present.) Bearing directly on the dependability of the total system is the long-term dependability of the output control device--typically an electromechanical relay--which must always respond properly to the control signal generated by the front end of the system. Finally, some self-correcting means must be built into the light curtain switch so as to compensate for any gradual drift or degradation which may occur in any of the components, especially the light sources and detectors.
The present invention addresses the dependability requirement and relates most directly to the safe operation of machine-guarding light curtains comprising: 1) an array of discrete photosources, each directing light onto a particular member of 2) a matching array of discrete photodetectors which are tied into 3) circuitry capable of producing a control signal that opens 4) an output control device whenever the flux of photosource light incident on one of the photodetectors decreases by more than some specific amount. The output control device--most commonly an electromechanical relay--is in turn part of a simple current loop emanating from the machine to be controlled; the interruption of that current loop initiates the machine's stopping mechanism. When viewed in this manner, it is clear why the light curtain system (including the relay) is often viewed as a "switch." Such light curtains serve as switch elements in control systems commonly used to halt (or, sometimes, to activate) machinery when an opaque object--e.g., a hand--moves between the photosource array and the photodetector array. One speaks of such an object as "penetrating the light curtain." Although the present invention will be seen to have applications wherever both sensing speed and high dependability are at a premium, it is sufficient for present purposes to describe it in the context of prior art related to machine-guarding light curtains.
The speed with which a light curtain system initiates a machine's stopping sequence depends largely on the type of components used, how quickly the circuit processes information, and how quickly the relay opens upon receiving the appropriate signal. With modern solid state devices, the factor determining how quickly the relay receives its "opening" signal is the method used to enhance system dependability; error checks and system verification take time. Ideally, however, that time should be much shorter than that which the relay takes to open after receiving the signal, in which case the overall response speed is still determined by the relay's speed.